                    [:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                    [:ft-lb :J]                #(* % (factors [:ft-lb :J]))
                    [:J :ft-lb]                #(/ % (factors [:ft-lb :J]))
+                   [:lbf :N]                  #(* % (factors [:lbf :N]))
+                   [:N :lbf]                  #(/ % (factors [:lbf :N]))
                    [:ft :m]                   #(/ % (factors [:m :ft]))
                    [:m :ft]                   #(* % (factors [:m :ft]))
                    [:lb :kg]                  #(/ % (factors [:kg :lb]))

(defn rand-unit [physical-quantity]
  (let [quantity-units-map {:density                [:lb-per-cu-ft :kg-per-m3]
                            :energy                 [:ft-lb :J]
                            :length                 [:ft :m]
                            :mass                   [:lb :kg]
                            :pressure               [:psi :kPa]

(defn convert-units-f [from to]
  (let [factors   {[:lb-per-cu-ft :kg-per-m3] 16.0185
                   [:ft-lb :J]                1.35582
                   [:lbf :N]                  4.44822
                   [:m :ft]                   3.28084
                   [:kg :lb]                  2.20462262185
                   [:psi :kPa]                6.894757293168361
